U.S. patent application number 14/111319 was filed with the patent office on 2014-08-07 for reflectivity-modulated grating mirror.
This patent application is currently assigned to DANMARKS TEKNISKE UNIVERSITET. The applicant listed for this patent is Il-Sug Chung. Invention is credited to Il-Sug Chung.
Application Number | 20140219301 14/111319 |
Document ID | / |
Family ID | 44533543 |
Filed Date | 2014-08-07 |
United States Patent
Application |
20140219301 |
Kind Code |
A1 |
Chung; Il-Sug |
August 7, 2014 |
REFLECTIVITY-MODULATED GRATING MIRROR
Abstract
The invention relates to vertical cavity lasers (VCL)
incorporating a reflectivity-modulated grating mirror (1) for
modulating the laser output. A cavity is formed by a bottom mirror
(4), an active region (3), and an outcoupling top grating mirror
(1) formed by a periodic refractive index grating region in a layer
structure comprising a p- and a n-doped semiconductor layer with an
electrooptic material layer (12) arranged there between. The
grating region comprises a grating structure formed by periodic
perforations to change the refractive index periodically in
directions normal to the oscillation axis. A modulated voltage (91)
is applied in reverse bias between the n- and p-doped layers to
modulate the refractive index of the electrooptic material layer
(12) and thereby the reflectivity spectrum of the grating mirror
(1). The reflectivity of the grating mirror (1) can be modulated
between a reflectivity with little or no out coupling and a
reflectivity with normal out coupling, wherein lasing in the VCL is
supported at both the first and the second reflectivity. As the out
coupling mirror modulates the output, the lasing does not need to
be modulated, and the invention provides the advantage of lower
power consumption at high modulation speeds.
Inventors: |
Chung; Il-Sug; (Ballerup,
DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chung; Il-Sug |
Ballerup |
|
DK |
|
|
Assignee: |
DANMARKS TEKNISKE
UNIVERSITET
Lyngby
DK
|
Family ID: |
44533543 |
Appl. No.: |
14/111319 |
Filed: |
May 9, 2012 |
PCT Filed: |
May 9, 2012 |
PCT NO: |
PCT/DK12/50157 |
371 Date: |
March 24, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61486930 |
May 17, 2011 |
|
|
|
Current U.S.
Class: |
372/28 |
Current CPC
Class: |
H01S 5/18386 20130101;
H01S 5/18316 20130101; H01S 5/18327 20130101; H01S 5/105 20130101;
H01S 5/18311 20130101; H01S 5/18366 20130101; H01S 5/0655 20130101;
H01S 5/18325 20130101; H01S 5/18302 20130101; H01S 5/3095 20130101;
H01S 5/18341 20130101 |
Class at
Publication: |
372/28 |
International
Class: |
H01S 5/183 20060101
H01S005/183 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2011 |
EP |
11166358.9 |
Claims
1. A vertical cavity laser with a reflectivity-modulated grating
minor, comprising: a cavity formed by a first and a second
reflector formed in different layers on a substrate and an active
region formed in the cavity, the cavity being arranged to support
light oscillation along an oscillation axis normal to the
substrate, wherein the first reflector is an outcoupling grating
mirror formed by a refractive index grating region in a layer
structure comprising a p-doped semiconductor layer and an n-doped
semiconductor layer with an electrooptic material layer arranged
there between, said grating region comprising a 1D or 2D grating
structure formed by a plurality of perforations so that a
refractive index changes periodically or nonperiodically in the
grating region in directions normal to said oscillation axis; and
electric contacts to apply electrical bias to the electrooptic
material layer and to the active region independently, wherein the
p-doped semiconductor layer and the n-doped semiconductor layer of
the grating minor act as electric contacts for the electrooptic
layer.
2-14. (canceled)
15. The vertical cavity laser according to claim 1, wherein the
cavity and the active region are selected to support lasing in the
vertical cavity laser at a predetermined wavelength.
16. The vertical cavity laser according to claim 15, wherein the
electrooptic material layer is configured to, upon application of a
first and a second reverse bias voltage between the p-doped
semiconductor layer and the n-doped semiconductor layer of the
grating minor, provide respective first and second reflectivity
spectra with different first and second reflectivities at the
predetermined wavelength.
17. The vertical cavity laser according to claim 15, wherein the
second reflector is a grating mirror made in a Si layer of a SOI
wafer and having a reflectivity of at least 99.9% at the
predetermined wavelength.
18. The vertical cavity laser according to claim 1, wherein the
electrooptic material is a quantum well semiconductor
structure.
19. The vertical cavity laser according to claim 1, wherein the
electrooptic material comprises a type-II heterojunction.
20. An optical interconnect comprising the vertical cavity laser
according to claim 1 configured to provide a light source.
21. A method for modulating the light emission from a vertical
cavity laser by modulating the reflectivity spectrum of an
outcoupling grating mirror of the vertical cavity laser, the method
comprising: providing a vertical cavity laser comprising an
outcoupling grating minor formed by a refractive index grating
region in a layer structure comprising a p-doped semiconductor
layer and an n-doped semiconductor layer with an electrooptic
material layer arranged there between, the grating region
comprising a 1D or 2D grating structure formed by a plurality of
perforations so that a refractive index changes periodically or
nonperiodically in said grating region in directions normal to an
oscillation axis of the vertical cavity laser; initiating laser
action in the vertical cavity laser at a predetermined wavelength;
and applying a modulated reverse bias voltage between the n-doped
semiconductor layer and the p-doped semiconductor layer to modulate
the refractive index of the electrooptic material layer to modulate
a reflectivity spectrum of the grating mirror between at least a
first and a second reflectivity spectrum providing different first
and second reflectivities at the predetermined wavelength,
respectively, wherein lasing in the vertical cavity laser is
supported at both the first and the second reflectivity.
22. The method according to claim 21, further comprising
maintaining the laser action in the vertical cavity laser
continuously during the modulation of the reflectivity spectrum of
the grating minor.
23. The method according to claim 21, wherein an electrical bias to
an active region the vertical cavity laser is not modulated during
the modulation voltage to the grating minor.
24. The method according to claim 21, further comprising receiving
one or more digitally-modulated electric signals and performing the
modulation of the reverse bias voltage between the p-doped
semiconductor layer and the n-doped semiconductor layer according
to the digital modulation of the electric signals so as to imply
the same modulation onto the reflectivity of the grating mirror and
thereby to the optical output signal of the vertical cavity
laser.
25. The method according to claim 21, wherein the first
reflectivity is in the interval of 99-99.5%, and wherein the second
reflectivity is at least 99.7%.
26. The method according to claim 21, wherein the electrooptic
material layer, the modulation voltages, and the predetermined
wavelength are selected so that the voltage modulation
predominantly modulates the refractive index in the electrooptic
material layer while absorption is substantially small.
27. The method according to claim 21, wherein the electrooptic
material comprises a quantum well semiconductor structure, type-II
heterojunction, or other structure, wherein the refractive index
modulation in the electrooptic material layer is a result of the
quantum-confined Stark effect (QCSE) or other effect and is thereby
wavelength dependent; and wherein the electrooptic material layer,
the modulation voltages, and the predetermined wavelength are
selected so that the voltage modulation predominantly modulates the
refractive index in the electrooptic material layer via QCSE or
other effect while an absorption is at least substantially small.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a vertical cavity laser
(VCL) incorporating a grating mirror of which the reflectivity can
be modulated to modulate the laser output power.
BACKGROUND OF THE INVENTION
[0002] In short-distance optical interconnects applications, low
energy consumption as well as high transmission speed of the
building block devices is becoming the key technological issues as
the data transmission bandwidth increases. Thus, the figure of
merit is energy consumption per transmitted bit. According to a
recent technology roadmap provided in D. A. B. Miller, "Device
requirements for optical interconnects to silicon chips",
Proceedings of the IEEE, vol. 97, p. 1166 (2009), a few 10s fJ/bit
is required in 2015-2020 for light transmitters of chip-level
optical interconnects.
[0003] As light emitter, vertical-cavity surface-emitting lasers
(VCSELs) are one of the preferable existing solutions. This is
because their fabrication technology is matured and their energy
consumption is much smaller than that of edge-emitting lasers due
to their small active material volume. To send a bit signal, output
light intensity of a light emitter should be modulated. There are
two schemes for modulating the output light intensity; direct
modulation and indirect (or external) modulation.
[0004] In the direct modulation scheme, the current injection to a
laser is modulated. This leads to the intensity modulation of the
output light. A state-of-the-art result is reported in Y.-C. Chang
and L. A. Colden, "Efficient, high-data-rate, tapered
oxide-aperture vertical-cavity surface-emitting lasers", IEEE
Journal of selected topics in quantum electronics, vol. 15, p. 704,
(2009). The transmission speed was 35 Gb/s, the energy consumption
excluding the RF driver circuitry was 12.5 mW, and the emission
wavelength was 980 nm. The demonstrated energy per bit of 357
fJ/bit (=12.5 mW/35 Gb/s) is remarkably small but is not sufficient
for the aforementioned applications. The weakness of this approach
is that it is difficult to further increase the speed or reduce the
energy consumption: Speed of a laser diode is decided by its
intrinsic response and circuit response. The intrinsic speed is
defined by -3 dB bandwidth of the intrinsic frequency response
which is proportional to relaxation oscillation frequency,
f.sub.r:
f r .varies. I - I th V p ( 1 ) ##EQU00001##
where I is the injection current, I.sub.th, threshold current, and
V.sub.p, modal volume. In order to obtain higher intrinsic speeds,
the injection current needs to be higher while the modal volume,
preferably smaller. In the demonstrated VCSEL, the modal volume is
not likely to be further reduced since the oxide aperture diameter
of 3 .mu.m is very small. Regarding the injection current, if one
increases the current for a higher intrinsic speed, it will result
in higher energy consumption. On the other hand, if one decreases
the current for smaller energy consumption, it will result in
slower intrinsic speed. Thus, it is difficult to further increase
the speed and decrease the energy consumption simultaneously in the
conventional VCSEL structure. One should also consider that high
injection current is detrimental to long-time stability of
small-volume lasers. The speed related to the circuit response is
mainly decided by the series resistance and capacitance of the
laser structure. In the demonstrated VCSELs, these parasitic terms
were already tightly suppressed. Thus, a significant improvement in
speed related to parasitic circuit terms is not expected.
[0005] In the externally modulated scheme, constant-intensity light
is generated in the laser part and the intensity modulation of this
light occurs in an integrated modulator part. Since no modulation
occurs in the laser part, current injection to the laser part can
be small, resulting in small energy consumption of the laser part.
Thus, if the energy consumption of the modulator part is small as
well, the energy consumption of the whole structure including both
the laser and modulator parts can be low.
[0006] A number of references disclose such an approach. For
example, in U.S. Pat. No. 7,593,436 part of the light-exiting
distributed Bragg reflector (DBR) includes an electrooptic
material. Thus, the reflectivity spectrum of this light-exiting DBR
can be modulated by modulating the reverse-biased voltage across
the electrooptic material. This modulation of reflectivity spectrum
leads to allowing and shutting the light emission, i.e., intensity
modulation of the light output. A state-of-the-art result obtained
by the inventors of the aforementioned invention is reported in V.
A. Shchukin, et al., "Ultrahigh-speed electrooptically-modulated
VCSELs: Modelling and experimental results," Proceeding of SPIE,
vol. 6889, 68890H, (2008). The energy per bit for the laser part
was 40-80 fJ/bit at injection currents of 1-2 mA while that for the
modulator part was about 100 fJ/bit. Here, the modulation speed was
40 Gbit/s and the lasing wavelength was about 960 nm. Thus, the
overall energy per bit was 140-180 fJ/bit which is fairly lower
than that of the direct modulation approach, but still needs
further reduction to meet the required specification. In addition,
another limitation that needs to be noted is that this approach of
using a DBR which embeds an electrooptic material is not feasible
for long wavelength VCSELs (wavelength 1310 nm): In order to obtain
a sufficient contrast in reflectivity at a lasing wavelength of
interest, the stopband widths of the passive DBR without an
electrooptic material and the active DBR with an electrooptical
material should be almost same. For long wavelength VCSELs,
dielectric DBR or GaAs/AlGaAs DBR with large stopband widths can be
used for passive DBRs. But, for active DBRs showing electrooptic
effect, one needs to use InP-based material which has a much
smaller stopband width than that of dielectric or GaAs/AlGaAs
DBRs.
[0007] Hence, an improved way of modulating the laser output would
be advantageous, and in particular a more efficient way of
providing modulated laser outputs at very fast modulation rates and
low energy consumption would be advantageous. In addition, a way
that works both at short and long wavelengths is advantageous.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide a
vertical cavity laser (VCL) and a method for modulating the output
of such a VCL that solve the above mentioned problems of the prior
art with limitations on modulation speed and energy
consumption.
[0009] Thus, the above described object and several other objects
are intended to be obtained in a first aspect of the invention by
providing a VCL with a reflectivity-modulated grating mirror
comprising:
[0010] a cavity formed by a first and a second reflector formed in
different layers on a substrate and an active region formed in the
cavity, the cavity being arranged to support light oscillation
along an oscillation axis normal to the substrate, wherein the
first reflector is an outcoupling grating mirror formed by a
refractive index grating region in a layer structure comprising a
p-doped semiconductor layer and an n-doped semiconductor layer with
an electrooptic material layer arranged there between, said grating
region comprising a 1D or 2D grating structure formed by a
plurality of perforations so that a refractive index changes
periodically or nonperiodically in the grating region in directions
normal to said oscillation axis; and
[0011] electric contacts to apply bias to the electrooptic material
layer and to the active region independently, wherein the p-doped
semiconductor layer and the n-doped semiconductor layer of the
grating mirror act as electric contacts for the electrooptic
layer.
[0012] In a second aspect, the invention provides a method for
modulating the light emission from a VCL by modulating the
reflectivity spectrum of an outcoupling grating mirror of the VCL,
the method comprising:
[0013] providing a VCL comprising an outcoupling grating mirror
formed by a refractive index grating region in a layer structure
comprising a p-doped semiconductor layer and an n-doped
semiconductor layer with an electrooptic material layer arranged
there between, the grating region comprising a 1D or 2D grating
structure formed by a plurality of perforations so that a
refractive index changes periodically or nonperiodically in said
grating region in directions normal to an oscillation axis of the
VCL;
[0014] initiating laser action in the VCL at a predetermined
wavelength; and
[0015] applying a modulated reverse-bias voltage between the
n-doped semiconductor layer and the p-doped semiconductor layer to
modulate the refractive index of the electrooptic material layer to
modulate a reflectivity spectrum of the grating mirror between at
least a first and a second reflectivity spectrum providing
different first and second reflectivities at the predetermined
wavelength, respectively, wherein lasing in the VCL is supported at
both the first and the second reflectivity.
[0016] The invention has the following advantages over VCLs
applying a reflectivity-modulated DBR such as US 2007/0291808 and
U.S. Pat. No. 5,408,486: [0017] The energy consumption related to
the reflectivity modulation can be significantly reduced. It is
because the voltage required to modulate the reflectivity of the
grating mirror can be much smaller than that of the DBR and the
energy consumption is proportional to the square of the voltage.
This allows for the design of VCLs with ultralow energy
consumption. Further details are discussed later in this section.
[0018] As discussed in the section `Background of the invention`,
the reflectivity-modulated DBR is difficult to be implemented in
wavelengths of 1310 and 1550 nm. The reflectivity-modulated grating
mirror applied in the invention can be universally implemented at
all wavelengths.
[0019] In the following, a number of further aspects, preferred
and/or optional features, elements, examples and implementations
will be described. Features or elements described in relation to
one embodiment or aspect may be combined with or applied to the
other embodiments or aspects where applicable. For example,
structural and functional features applied in relation to the VCL
may also be used as features in relation to the method for
modulating the emission of a VCL by proper adaptation and vice
versa. Also, explanations of underlying mechanisms of the invention
as realized by the inventor are presented for explanatory purposes,
and should not be used in ex post facto analysis for deducing the
invention.
[0020] The perforations are holes extending through the entirety of
the layer structure, and thereby through the p/n-doped
semiconductor layers and the electrooptic layer between them. The
perforations are preferably filled by air or any other electrically
insulating medium with a refractive index substantially different
from that of the layer structure. The perforations are formed so
that a refractive index changes periodically or nonperiodically in
the grating region in directions normal to said oscillation axis,
in order to get transverse mode confinement in the laser. That also
nonperiodic gratings may be used for this purpose is a
understanding. In the present context, nonperiodic can be
modulation of periodicity or truly non-periodic.
[0021] The basic structure of a VCL is the cavity between the two
reflectors formed in layers on a substrate, and being arranged to
support light oscillation along an oscillation axis normal to the
substrate. Other laser types may be formed in layered structures
without having vertical cavities, such as edge-emitting lasers.
Such lasers are based on a very different design with an in-plane
oscillation axis and thus represent a different technical field
than the present invention. In preferred embodiments of the
invention, the laser couples out radiation to the air or a
waveguide through the top reflector, making the laser in these
embodiments vertical-cavity surface-emitting laser (VCSEL, a
special group of VCLs). Other types of laser may be surface
emitting without being VCLs, e.g. if the outcoupling is based on
emission of scattered light from the cavity. An example of a
surface emitting laser formed in a layered structure that is not a
VCL can be found in U.S. Pat. No. 6,826,223.
[0022] The wavelength-dependent reflectivity spectra of the
reflectors and the optical gain of the active region are selected
to support lasing in the VCL at a predetermined wavelength, also
referred to as the laser wavelength, preferably between 650 nm-2000
nm, such as preferably around 850 nm, 980 nm, 1060 nm, 1310 nm, or
1550 nm.
[0023] Also, in preferred embodiments, the second reflector has a
reflectivity of at least 99.9%, such as preferably 99.9%. The
second reflector may be another grating mirror made in the Si layer
of a SOI wafer, or a DBR, depending on applications and
wavelengths.
[0024] An example of a prior art VCSEL using two dimensional (2D)
grating mirrors can be found in e.g. US 2007/0201526. This
structure differs from the present invention in several ways; one
of them being that reflectivity of the grating mirrors cannot be
modulated. None the less, the technology is similar and extensive
references to VCSEL technology is made throughout the present
description. The invention is thereby also advantageous as it
applies matured VCSEL technologies exhibiting low power
consumption, eminent single-mode property and good mode control in
general. Further, the use of VCSEL technology allows for
uncomplicated packaging.
[0025] The reflectivity-modulated grating mirror according to the
invention functions as a reflector to establish the VCL cavity. The
high reflectivity of grating mirrors is related to the resonant
coupling between vertically incident free space modes and
laterally-propagating modes of the grating. Vertically incident
light is diffracted when it arrives at the grating, and due to
sub-wavelength scale of the grating, all diffractions higher than
the zero-th order occur in the direction of the grating plane.
These are coupled to the grating modes. After some propagation in
the grating, the grating modes are radiated back to the free space
mode. This resonant coupling results in high reflectivity.
[0026] As previously indicated, the electrooptic material applied
in the grating region is preferably QW or type-II heterojunction
material. The modulation of the reflectivity of the grating mirror
according to the invention is preferably based on the quantum
confined Stark effect (QCSE). The reverse bias results in a strong
external electrical field applied along the direction of the
quantum confinement (here perpendicular to the layers) of the
electrooptic material layer (quantum-confined system). The reverse
bias setup means that virtually no current will flow through the
electrooptic material. Changing the bias voltage results in a shift
in both the wavelength and the intensity of the optical absorption
peak due to the Stark effect, and thereby also in a shift in the
refractive index (due to Kramers-Kronig relationship between the
real and imaginary parts of the dielectric function of the
medium).
[0027] The refractive index change of the grating mirror alters the
resonance condition during the reflection process. The reflectivity
spectrum is thereby also changed according to the grating mirror
reflection mechanisms described above.
[0028] The relative shifts in absorption and refractive index are
wavelength-dependent. According to the working principle of the
present invention, the electrooptic material and the predetermined
wavelength are preferably selected so that a reverse bias voltage
modulation can be selected to modulate in particular the refractive
index in the electrooptic material at the predetermined wavelength,
while the absorption is kept substantially low.
[0029] In a preferred embodiment, the electrooptic material is QW
semiconductor structures or type-II heterojunctions, with the
semiconductor junctions designed or selected to provide the desired
refractive index modulation and negligible absorption at the laser
wavelength.
[0030] A QW structure can be formed in the layer structure by a
thin semiconductor material layer (e.g. GaAs) sandwiched between
two layers with a wider bandgap (e.g. AlAs). The QW structures may
for example be grown by using a molecular beam epitaxy (MBE) or a
metal organic chemical vapor deposition (MOCVD) equipment, and can
be very thin, typically 6-10 nm thick. Electrons and holes are
confined within the quantum well.
[0031] A type-II heterojunction formed in the layer structure
comprises two different semiconductor materials. Heterojunction
structures also can be grown by using a MBE or a MOCVD equipment.
In the case of zero electric field, electrons and holes are
separately and weakly confined on the opposite sides of the
heterojunction.
[0032] It is an advantage of the reflectivity-modulated grating
mirror that the layer over which the electric filed is applied,
i.e. the thickness of undoped electrooptic material layers between
the p/n-doped contact layers, can be made very thin. This means
that the bias voltage, V needed to produce the necessary electric
field strength for refractive index modulation, can be small. Since
this grating mirror structure with an electrooptic material is a
capacitor structure, the energy consumption for modulating the
reflectivity, P.sub.mod is given by the following equation;
P.sub.mod=1/2CV.sup.2 (2)
[0033] Here, C is the capacitance of the grating mirror. In the
thin grating mirror structure applied in the invention, the
required voltage V can be as small as e.g. 0.2 V which is
approximately 5 times smaller than the prior art employing the
reflectivity-modulated DBR, e.g., US 2007/0291808 and U.S. Pat. No.
5,408,486. Also, the capacitance C can be e.g., 7 fF with a
7-.mu.m-diameter doped region, which is approximately 2 times
larger than the prior art, e.g., employing a DBR-based modulator
with seven 3-QW pairs and a 7-.mu.m-diameter doped region. Thus,
the energy consumption for light modulation may be 10 times smaller
than in the prior art. This allows for the design of VCLs with
ultralow energy consumption.
[0034] The modulation of the voltage across the electrooptic
material is preferably between at least the first and second
voltages selected corresponding to the desired first and the second
reflectivity values. The first voltage is preferably selected so
that a reflectivity at the laser wavelength in the first
reflectivity spectrum is a normal value for an outcoupling mirror
in VCLs to provide a typical light emission intensity for the
intended application while maintaining lasing, such as preferably a
reflectivity in the interval of 99 to 99.5%. The second voltage is
preferably selected so that a reflectivity at the laser wavelength
in the second reflectivity spectrum turns off or reduces the
emission significantly to provide a binary intensity modulation
between the two emission levels, such as preferably a reflectivity
of higher than 99.7%.
[0035] The laser action in the VCL is maintained and preferably
kept unchanged during the modulation of the reflectivity spectrum
of the grating mirror. Thus, the first and second reflectivity
values need to be chosen sufficiently high so that the photon
density in the laser cavity are kept sufficiently high to sustain
lasing regardless of the reflectivity modulation. Otherwise, the
modulation speed of emitted light intensity will be also influenced
by the frequency response of the laser cavity. That is, it would
fall into the direct modulation regime. As discussed in the section
`Background of the invention`, the direct modulation speed of a
VCSEL laser cavity is difficult to exceed 40 Gb/s. In principle,
the speed response of the electrooptic grating mirror applied in
the invention may exceed 100 GHz. Thus, 150 Gb/s would potentially
be possible.
[0036] It is also preferred that the electrical bias to the active
region the VCL is not modulated during the modulation of the
voltage to the grating mirror.
[0037] In a further aspect, the invention provides optical
interconnects comprising one or more VCLs according to the first
aspect of the invention for generation of optical data signals
based on received electric data signals. The optical interconnects
may be implemented in on-chip level as well as the off-chip level.
Such optical interconnects may solve most limitations of the
current electrical interconnects such as energy consumption
problem, crosstalk, speed limitation, channel density limitation,
and high-speed clocking limitation. Compared to existing laser
sources for chip-level optical interconnects, the laser sources
according to the present invention involve the advantages of higher
obtainable data rates and much lower energy consumption.
[0038] The basic idea of the invention is to make a grating mirror
of which the reflectivity can be modulated, and to apply this
reflectivity-modulated grating mirror to modulate the emission of a
VCL. By modulating an electrical field of an electrooptic material
in the grating mirror, the refractive index and ultimately the
reflectivity of the grating mirror can be modulated very fast based
on the QCSE. As only the electrical field is to be modulated with
virtually no current flow, and as the layer over which it is
applied can be made very thin, very low power consumption and high
modulation speeds can be achieved.
[0039] These and other aspects of the invention will be apparent
from the following description with reference to the described
embodiments.
BRIEF DESCRIPTION OF THE FIGURES
[0040] Embodiments of the invention will now be described in more
detail with regard to the accompanying figures. The figures show
one way of implementing the present invention and is not to be
construed as being limiting to other possible embodiments falling
within the scope of the attached claim set.
[0041] FIG. 1A is a schematic cross-sectional side-view of an
example device of the invention. The cross-section is along the
line AB designated in FIG. 1B.
[0042] FIG. 1B is the top view of the device in FIG. 1A.
[0043] FIG. 2 is the cross-sectional side-view of an example device
made without wafer bonding.
[0044] FIG. 3 is the cross-sectional side-view of an example device
with a bottom Si grating mirror.
[0045] FIG. 4 is the cross-sectional side-view of an example device
with an oxide aperture.
[0046] FIG. 5A is the energy band structure of an electrooptic QW
with no electric field applied.
[0047] FIG. 5B is the energy band structure of the QW of FIG. 5A
with a reverse bias voltage applied.
[0048] FIG. 5C is the absorption spectra of the QW of FIG. 5A with
(dotted line) and without (solid line) a reverse bias voltage. The
.lamda..sub.0 designates the predetermined lasing wavelength.
[0049] FIG. 5D is the real refractive index spectra of the QW of
FIG. 5A with (dotted line) and without (solid line) a reverse bias
voltage.
[0050] FIG. 6A is the energy band structure of an electrooptic
heterojunction structure with no electric field applied.
[0051] FIG. 6B is the energy band structure of the heterojunction
structure of FIG. 6A with a reverse bias voltage applied.
[0052] FIG. 6C is the absorption spectra of the heterojunction of
FIG. 6A with (dotted line) and without (solid line) a reverse bias
voltage. The .lamda..sub.0 designates the predetermined lasing
wavelength.
[0053] FIG. 6D is the real refractive index spectra of the
heterojunction of FIG. 6A with (dotted line) and without (solid
line) a reverse bias voltage.
[0054] FIG. 7 is a cross-section of an outcoupling grating mirror
embedding an electrooptic material according to an embodiment of
the invention.
[0055] FIG. 8 illustrates a 2D grating structure of a grating
mirror according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0056] FIGS. 1A and 1B show cross-sectional side- and top views of
an example of the invention, respectively. In this embodiment, the
device consists of a grating mirror 1 having an embedded
electrooptic material 12 and a grating structure 10, an air gap 21
(or sacrificial layer 2), an active region 3, and a DBR 4 all held
by substrate 5. Light is generated in the active material 32 which
is typically QWs, is vertically amplified between the grating
mirror 1 and the DBR 4, and is emitted through the grating mirror
1. The forward bias 92 for light generation is applied between the
p contact 84 and the n contact 83. The injected current flows
through the n-doped layer 34, the tunnel junction 35, the p-doped
layer 33, the active material 32, and the n-doped layer 31. The
reflectivity of the grating mirror 1 is modulated to modulate light
emission. The reverse bias 91 is applied between the p contact 81
and the n contact 82. Thus, a strong electric field is induced in
the electrooptic material 12 between the p doped layer 11 and the n
doped layer 13. Modulating the strength of the electric field by
modulating the reverse bias voltage 91, results in the modulation
of the refractive index of the electrooptic material 12, due to the
QCSE. This refractive index modulation leads to the modulation of
the reflectivity of the grating mirror 1 and that of light
emission.
[0057] Injected currents for light generation are confined by the
tunnel junction 35, and the optical mode confinement is obtained by
the tunnel junction 35 as well as the low-refractive-index trench
36, preferably air trench. The optical mode confinement due to the
tunnel junction 35 is a combination of several effects; the tunnel
junction 35 can be designed to have a higher refractive index than
the surrounding material. In addition, higher current density
within the tunnel junction 35 further increases its refractive
index and higher temperature around the tunnel junction 35 also
increases the refractive index around it. The overall higher
refractive index around the tunnel junction 35 provides optical
confinement around the tunnel junction. The low-refractive-index
trench 36 further strengthens the optical confinement. However, the
tunnel junction 35 alone can provide the optical confinement so
that in some device designs, the low-refractive-index trench 36 may
be omitted.
[0058] Based on the descriptions and illustrations herein, detailed
procedures of fabricating this device will be apparent to the
people skilled in conventional VCSEL technology. The example device
described in relation to FIG. 1 presumes wafer bonding of a passive
DBR part and an active part. The passive DBR part includes the DBR
4, preferably undoped GaAs/AlGaAs DBR and the substrate 5,
preferably GaAs substrate. The active part includes epitaxial
layers for the grating mirror 1, the sacrificial layer 2, and the
active region 3. The active part is preferably made of InP-related
materials for laser wavelength of 1310 and 1550 nm. This active
part is grown from upper layers to lower layers by using an epitaxy
growth equipment; layer 11 is grown first and layer 34 is grown
last. The low-refractive-index trench 36 is formed before wafer
bonding. This procedure is the same as in the fabrication of
conventional long-wavelength VCSELs. After the wafer bonding, the
substrate for the growth of the active part, preferably InP
substrate is removed and the mesas, grating, and contacts are
formed.
[0059] In this example device, the contacts 83 and 84 for light
generation are intra-cavity contacts. However, if relevant or
necessary, the p-contact 84 may be formed at the bottom of the
substrate 5. In this case, the DBR 4 and the substrate 5 should be
doped appropriately. In the grating mirror 1, the positions of the
p-doped layer 11 and the n-doped layer 13 can be swapped. Then, the
direction of the reverse bias 91 needs to be changed accordingly.
In this swapped case, the contact 82 is n-contact and the layer 13
is n-doped material. Thus, the n-contact 82 can be merged with the
n-contact 83 if relevant or necessary. For this the sacrificial
layer 2 should be n-doped.
[0060] In the following in FIGS. 2-4, a number of additional
embodiments are described, with different configurations of the
bottom mirror 4 and the active region 3. The configuration of the
grating mirror 1 and the air gap 21 or sacrificial layer 2 is
similar to that described in relation to FIG. 1 above. Features
referred to by the same reference numerals as in FIG. 1 are similar
to those described in relation to FIG. 1.
[0061] In FIG. 2, another example is illustrated. In this
embodiment, the bottom DBR 104 is formed by growth or deposition
rather than wafer bonding. After forming epitaxial layers 31 to 34
successively by using an epitaxial growth equipment, the trench 36
for optical confinement may or may not be formed. Then, the bottom
DBR 104 is formed by using the same epitaxial growth equipment or
is deposited by using a dielectric deposition equipment. Then, the
carrier substrate 105 is bonded. The active part including the
grating mirror, the sacrificial layer, and the active region 103
can be either InP-related materials or GaAs-related materials,
depending on the laser wavelength. The DBR 104 can be InP-related
materials, GaAs-related materials, and dielectric materials.
[0062] In FIG. 3, another example is illustrated. In this
embodiment, the bottom mirror is another grating mirror formed in a
Si layer 251 of a silicon-on-insulator (SOI) substrate 250. After
forming the active part including layers for the grating mirror and
the active region 3, the trench 36 may or may not be formed for the
optical mode confinement. Then, SiO.sub.2 layer 241 can be
deposited on the active part which can be similar to the active
parts described in relation to FIG. 1 or 2. Thereafter, the active
part with the SiO.sub.2 layer 241 deposited can be wafer bonded to
the SOI wafer 250 with the Si grating formed.
[0063] In the embodiments illustrated in FIGS. 1 to 3, the tunnel
junction 35 is preferably located below the active material 32.
However, it can also be located above the active region. In this
case, the position of the p-doped layer 33 needs to be accordingly
changed, which will be straightforward to the people skilled in
conventional VCSEL technology.
[0064] In FIG. 4, another example is illustrated. This embodiment
is similar to that of FIG. 1 or 2, but all epitaxial layers can be
grown using preferably GaAs-related materials without any wafer
bonding or re-growth. The growth order is from lower layers and
upper layer, i.e., the DBR layers in bottom DBR 304 first and the
layers 13, 12, and 11 of the grating mirror last. In this
embodiment, the injected current flows through the p-doped layer
333, the active material 332, and the n-doped layer 331. The oxide
aperture 335 provides carrier confinement as well as optical mode
confinement, and can be formed by wet oxidation. If necessary, the
n-contact 83 can be positioned at a different position, e.g., at
the bottom of substrate 305. In that case, the bottom DBR 304 and
the substrate 305 should be n-doped. If necessary, another oxide
aperture can be added below the active material 332. Other
variations of this structure known in the literature of VCSELs may
be applied if relevant or necessary.
[0065] Modulation of the Reflectivity
[0066] The refractive index of the electrooptic material 12
embedded in the top grating mirror 1 can be modulated by modulating
the strength of electric field applied over the electrooptic
material i.e. by modulating the reverse bias voltage 91. The
electrooptic effect preferably employed here relies on the quantum
confined Stark effect. However, other electrooptic material relying
on another effect can be also used if the amount of its refractive
index modulation is comparable to that from the quantum confined
Stark effect.
[0067] Here, two preferable electrooptic materials are described;
QW and type-II heterostructure. A QW structure consists of a thin
layer with a smaller energy bandgap and two large-bandgap materials
which surrounds the smaller-bandgap layer. As illustrated in FIG.
5A, when there is no (or only a weak) external electric field
applied through the QW (E.about.0), the electrons 501 and holes 502
are confined within the energy wells in the conduction and valence
bands, respectively. The electron and hole states are separated by
energy difference 503. When a strong external field is applied
(E=E.sub.ext), the energy structure is accordingly inclined as
illustrated in FIG. 5B. The energy level difference 513 between the
electron state 511 and the hole state 512 becomes smaller than the
unbiased energy difference 503; this results in the shift of the
peak absorption wavelength toward the longer wavelengths as shown
in FIG. 5C, where the solid and dotted curves represents the
absorption spectra of the QW without and with an external field,
respectively. The peak absorption value of the QW with an external
field (dotted curve) is smaller than that without an external field
(solid curve). This is attributed to, that an increased spatial
separation 514 of the electron and hole distributions resulting
from the external field leads to a weaker transition strength
between them, which again results in a weaker absorption. This
change of absorption spectrum accompanies the change of a real part
of the refractive index as shown in FIG. 5D, which is explained by
the Kramers-Kronig relation. Thus, the refractive index value of
the electrooptic material layer 12 of the grating mirror at a laser
emission wavelength .lamda..sub.0 can be modulated as a function of
the applied field strength.
[0068] The other electrooptical material, type-II heterostructure,
consists of two materials with difference band energies. As
illustrated in FIG. 6A, when no (or only a weak) external field is
applied (E".about.0), the electron distribution 601 and the hole
distribution 602 have different energy levels with an energy
difference 603 and are spatially separated by a distance 604. When
an external field is applied (E=E.sub.ext), the energy band becomes
inclined as illustrated in FIG. 6B. Now, the spatial separation 614
becomes smaller than 604 while the energy difference 613 becomes
larger than 603. In FIG. 6C, the absorption spectra without an
external field (solid curve) and with an external field (dotted
curve) are presented. When there is no external field, the
absorption is weak due to the large spatial separation 604. When an
external field is applied, the absorption spectrum peak shifts
toward shorter wavelength due to the increased energy difference
613 and the peak value becomes larger due to the smaller spatial
separation 614. As a result, a refractive index modulation of the
electrooptic material layer 12 of the grating mirror at the
emission wavelength .lamda..sub.0 can be obtained as shown in FIG.
6D.
[0069] As illustrated by FIGS. 5C-D and 6C-D, the relative shifts
in absorption and refractive index are wavelength dependent, and
the electrooptic material and the emission wavelength can be
selected so that the modulation of the applied external field
strength will result, in particular, in the modulation of the
refractive index while the absorption is kept substantially low.
This condition is for example fulfilled at the wavelength
.lamda..sub.0 as indicated FIGS. 5C-D and 6C-D.
[0070] As described previously, the refractive index change of the
electrooptic material layer of the grating mirror alters the
resonance condition during the reflection process. Thus, the
reflectivity spectrum is also modulated by the applied external
field, and thereby also the reflectivity of the grating mirror 1 at
the (predetermined) emission wavelength of the laser.
[0071] The reverse bias 91 is modulated between a first and a
second voltage selected to correspond to a first and a second
reflectivity values of the grating mirror 1 at the emission
wavelength of the laser. FIG. 7 is a cross-section of an
outcoupling grating mirror 1 embedding an electrooptic material 12
according to an embodiment of the invention. Arrows 700 and 701
illustrate light impinging at and being reflected by the grating
mirror 1, respectively. Arrow 703 illustrates light being
transmitted by the grating mirror 1 and thereby coupled out of and
the resonator cavity of the laser, resulting in an emission.
[0072] The first voltage is preferably selected so that a
reflectivity value at the laser wavelength in the first
reflectivity spectrum is in the interval of 99-99.5%, resulting in
an emission 703 sufficient for the intended application while
providing a reflection 701 large enough to maintain lasing action
in the light generating part. The second voltage is preferably
selected so that a reflectivity value at the laser wavelength in
the second reflectivity spectrum turns off or reduces the emission
703 to provide a binary modulation between the two emission levels,
such as a reflectivity value higher than 99.7%. The voltage values
can be decided based on the numerical simulation results, i.e., the
reflectivity spectrum of a grating mirror as a function of
refractive index change of the electrooptic material, and the
refractive index change of the electrooptic material as a function
of applied voltage.
[0073] In a preferred embodiment, the VCL is used as a light source
in optical interconnects, where it serves to convert one or more
received digitally-modulated electrical signals into a
digitally-modulated optical signal. For this purpose, the reverse
bias voltage signal will be modulated according to the one or more
received digitally-modulated electrical signals.
[0074] Optical Confinement
[0075] A number of known techniques are available for defining the
position of the optical mode in a VCL, also referred to as the
transverse mode confinement or optical confinement; several of
these are illustrated in connection with FIGS. 1 through 4. As VCLs
according to embodiments of the invention have an outcoupling
grating mirror that is different from known VCLs, in that it is
reflectivity modulated. A general discussion of the optical
confinement techniques most suitable for VCLs according to
embodiments of the invention is provided in the following.
[0076] Typically, the carriers and increased temperature from the
current confinement will provide a weak optical confinement also
referred to as thermal lensing effect. However, to achieve the
desired mode confinement, one or more of the following techniques
can be applied.
[0077] The mode confinement in conventional optical waveguides is
achieved by having a core with a high refractive index surrounded
by a cladding with a lower refractive index. This results in a
waveguide based on the principle of total internal reflection.
[0078] In an optical resonator such as a VCL, the shift of a cavity
resonance wavelength corresponds to an effective step in refractive
index, .DELTA..lamda./.lamda.=.DELTA.n/n, due to the wavelength
dependence of the dielectric function of the materials, see also G.
R. Hadley, "Effective index model for vertical-cavity
surface-emitting lasers," Optics Letters, vol. 20, No. 13, p. 1483
(1995). As a result, lateral mode confinement in a VCL can be
accomplished by having a central (core) region with a long cavity
resonance-wavelength surrounded by an outer (cladding) region with
a short cavity resonance-wavelength.
[0079] A nonperiodic grating can also be used to define transverse
mode. Non-periodic grating can be designed to provide focusing to
the reflected beam while keeping high reflectivity, see J. Li, et
al., "Strong optical confinement between nonperiodic flat
dielectric grating," Physical Review Letters, vol. 106, p. 193901
(2011). The nonperiodic grating scheme can be introduced in the
grating mirror embedding the electrooptic material or in the bottom
grating mirror without the electrooptic material, e.g., in the
bottom Si grating mirror in FIG. 3.
[0080] Gain/Current Confinement
[0081] Several suitable optical gain media as well as a number of
known techniques for defining the position of the optical gain
region in the gain medium (typically referred to as current
confinement) are available for VCLs. The active region can
typically be a multiple QW material layer (a different layer than
the electrooptic material layer of the first grating mirror). The
current confinement can be provided by a current aperture formed by
proton implantation above the QW layer. The implanted region
becomes insulating, working as a current aperture and defining the
gain region. Alternatively, the optical gain region can be defined
by a dielectric aperture, preferably an oxide aperture formed in or
near the layer of the active region. The optical gain region can
also be defined by a tunnel junction. In a tunnel junction, highly
n-doped and p-doped thin layers are put together, and the current
flows only through this junction. The various forms of providing an
active region and current confinement are considered known to the
person skilled in the art of designing VCLs.
[0082] Grating Region and Grating Mirror
[0083] The grating mirror 1 comprises a one-dimensional (1D) or 2D
periodic refractive index grating structure 10 formed in the
electrooptic material layer 12 and in the p- and n-doped layers 11
and 13. A 1D grating structure 10 is illustrated in FIG. 1B and a
2D grating structure 10 is illustrated in FIG. 8. The grating
structure 10 may be formed by removing the material in layers 11,
12, and 13, e.g. by using matured processing techniques to form
perforations. The perforations can be left empty (i.e. air filled)
or could potentially be filled with a low refractive index
material. In an alternative approach, the grating structure 10 is
formed by other modification of these layers.
[0084] Different 1D and 2D patterns of the periodic refractive
index grating structure 10 are shown in FIGS. 1B and 8. In both
cases, the pattern are periodic to have photonic bandgap (PBG)
modes. For transeverse mode confinement, the grating can be
non-periodic. From a design point of view, the important thing is
which pattern gives desirable PBG mode dispersion and the resultant
mirror characteristics. From a fabrication point of view, the
mechanical stability of thin grating layer structure, the
fabrication feasibility, and fabrication cost need to be considered
to choose the pattern.
[0085] The periodicity, thickness, refractive index, air-filling
ratio, and lattice structure (e.g., triangular or square lattice)
of a grating structure determines its PBG mode dispersion. In the
following, preferred possible ranges of the aforementioned design
parameters of grating structure are listed. Other ranges and values
can be used if they lead to desirable PBG mode dispersion.
[0086] The periodicity of grating pattern, either 1D or 2D, ranges
from 0.4.lamda. to 0.8.lamda. where .lamda. is the lasing
wavelength of interest. The optical thickness of a grating layer
can typically range from 0.5.lamda. to 1.6.lamda.. The optical
thickness of a layer is defined as the physical thickness of the
layer multiplied by the refractive index of the layer. The air
filling ratio typically ranges from 20% to 85%. The air filling
ratio is defined as a fraction of air (or other low index material)
area among the area of one grating period.
[0087] The origin of the high reflection of the grating mirror is
explained in terms of the modes involved. The involved modes are a
vertically resonant mode supported by the VCL cavity and
laterally-propagating PBG modes of the grating mirror. These PBG
modes are Eigenmode of the periodic refractive index grating
structure 10. The light in the vertical cavity mode is diffracted
when incident to the grating; the part of diffracted light is
coupled to the horizontal grating PBG modes; the light in the
grating modes is coupled back to the cavity mode. This coupling
process may lead to resonance which again leads to a high
reflectivity. The number of grating modes involved in the coupling
can be one or plural, depending on the design. No coupling loss is
expected provided that the whole grating structure is exactly
periodic and has no absorption.
APPLICATIONS
[0088] The laser comprising a reflectivity modulated outcoupling
grating mirror according to the invention is promising in
applications where high speed modulation and/or low energy
consumption is required. In short-distant optical interconnects
applications such as chip- and off-chip level optical interconnects
for computers, ultralow energy consumption per sending a bit signal
is highly required. There has been no light source with superior
energy-consumption-per-bit value as well as feasible fabrication
and mW-level output power, reported yet. Thus, the invention has
potential to be a breakthrough solution. In conventional optical
communication applications, the invention can be competitive over
DFB laser and conventional long wavelength VCSELs. Against
conventional DFB lasers, the laser according to the invention will
consume much less energy. Against conventional long wavelength
VCSELs, the laser according to the invention will have higher
modulation speed, lower energy consumption, and lower material
cost. State-of-the-art VCSEL demonstrates 350 fJ/bit at short
wavelength. The laser according to the invention can achieve lower
than 100 fJ/bit both at short and long wavelengths. If metamorphic
growth of a bottom DBR or a bottom Si grating on a SOI wafer is
employed, the wafer cost can be half as small as the conventional
long wavelength VCSEL technologies.
[0089] Although the present invention has been described in
connection with the specified embodiments, it should not be
construed as being in any way limited to the presented examples.
The scope of the present invention is to be interpreted in the
light of the accompanying claim set. In the context of the claims,
the terms "comprising" or "comprises" do not exclude other possible
elements or steps. Also, the mentioning of references such as "a"
or "an" etc. should not be construed as excluding a plurality. The
use of reference signs in the claims with respect to elements
indicated in the figures shall also not be construed as limiting
the scope of the invention. Furthermore, individual features
mentioned in different claims, may possibly be advantageously
combined, and the mentioning of these features in different claims
does not exclude that a combination of features is not possible and
advantageous.
REFERENCES
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[0091] Chang and Colden, "Efficient, high-data-rate, tapered
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[0093] V. A. Shchukin, et al., "Ultrahigh-speed
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[0098] G. R. Hadley, "Effective index model for vertical-cavity
surface-emitting lasers," Optics Letters, vol. 20, No. 13, p. 1483
(1995).
[0099] J. Li, et al., "Strong optical confinement between
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106, p. 193901 (2011).
* * * * *